Oh for heaven's sake buy the circular fish!! They are so much fun.
Granted you will likely not be using the lens regularly. But get it, play with it for awhile and see what happens. If you don't like it, sell it on Ebay.
I have this particular Sigma 8/3.5 EX DG and like it very much. Well made and Sigma finishes the lenses nicely.
There is a removable front ring which I think is meant to be a lens shade. Anyway, on an FX camera if you remove the ring you get a larger circle.
Practicing circular composition is an interesting effort.
I use the Sig8 in one of my favorite series. Here is Man in Kitchen with Smiling Pet Cat
(originally shown on Fotozones).